1. Field of the Invention
The present invention relates to solid-state imaging devices. In particular, the present invention relates to CMOS solid-state imaging devices which transfer signal charge to floating diffusions, and methods for fabricating the solid-state imaging devices. The present invention also relates to electronic apparatuses employing the solid-state imaging devices.
2. Description of the Related Art
Solid-state imaging devices are generally classified into CCD (charge coupled devices) solid-state imaging devices and CMOS (complementary metal oxide semiconductor) solid-state imaging devices. In a CCD solid-state imaging device, a high drive voltage is necessary to transfer signal charge, and a high supply voltage is necessary compared to a CMOS solid-state imaging device. Thus, CMOS solid-state imaging devices are more advantageous than CCD solid-state imaging devices in view of power consumption.
Thus, CMOS solid-state imaging devices, which are more advantageous than CMOS solid-state imaging devices, have been in wide use as solid-state imaging devices for mobile apparatuses such as mobile phones equipped with cameras and PDAs (personal digital assistants).
A CMOS solid-state imaging device includes a photodiode and is composed of a photodetector for generating signal charge upon receiving light, a floating diffusion receiving the signal charge generated by the photodetector, and a plurality of MOS transistors. The MOS transistors include a transfer transistor, a reset transistor, an amplification transistor, and, as necessary, a selection transistor. These MOS transistors are connected predetermined wiring layers in a multi-wiring layer. In the CMOS solid-state imaging device, signal charge generated and accumulated in the photodetector is transferred by the transfer transistor to the floating diffusion on a pixel by pixel basis. The signal charge read by the floating diffusion is amplified by the amplification transistor and selectively output to a one of vertical signal lines formed in the multi-wiring layer.
Meanwhile, in such a CMOS solid-state imaging device, a larger aperture above a photodiode serving as a photodetector is more desirable to efficiently collect light incident onto the photodiode.
On the other hand, photoelectric conversion also occurs in the floating diffusion if it receives light while reading signal charge transferred from the photodetector, which results in noise. Therefore, it is desirable that the floating diffusion is shielded from light.
In a CMOS solid-state imaging device according to the related art, a floating diffusion is shielded from light using a multi-wiring layer disposed above the substrate. However, since pixels are formed using CMOS processes as in the case of the other peripheral circuits, the wiring layers may not be disposed so as to be immediately above the floating diffusion, as compared to the case of a CCD solid-state imaging device. Thus, it is not possible to decrease the distance between the wiring layers constituting a light-shielding film and a floating diffusion and to prevent light leakage into the floating diffusion.
In view of the above problem, Japanese Unexamined Patent Application Publication No. 2004-140152 discloses a CMOS solid-state imaging device in which a light-shield is formed using a multilayer gate electrode film. In this technique, since the light-shield is arranged immediately above the floating diffusion, leakage of light into the floating diffusion may be suppressed. However, to form the gate electrode film in a multilayer structure, the gate electrode film is made of silicide, which leads to complicated fabrication processes. In addition, it is difficult to form a small aperture for a contact region due to unevenness of the gate electrode. Thus, it is necessary for a floating diffusion to have a large area in order to form the contact region. Moreover, interference between the gate electrode layers due to a large parasitic capacitance between the gate electrode layers is also a matter of concern.
Japanese Unexamined Patent Application Publication No. 2004-71931 discloses a technique in which gate electrodes of a floating diffusion and an amplification transistor are electrically connected without wiring. This increases the degree of freedom in layout of wiring layers and thus makes it possible to increase the size of the aperture of the photodetector. However, according to Japanese Unexamined Patent Application Publication No. 2004-71931, a floating diffusion is shielded from light using a wire arranged in a multi-wiring layer disposed in an upper layer. This allows light to travel between the light-shielding film and the floating diffusion, resulting in insufficient light shielding performance.